1. Field of the Invention
The present invention relates to an encoder.
2. Description of the Related Art
In order to improve the detecting precision of an absolute encoder like an incremental encoder, Japanese Patent Laid-Open No. (“JP”) 2009-198318 forms such an incremental pattern on each track on a scale that a shape or pitch of the pattern periodically changes at a modulation period in a length measuring direction. Thereby, the incremental pattern has not only information of a relative position but also modulation information of an absolute position using a repetitive modulation period. As a result, the absolute position of the scale can be highly precisely detected using information of a highly precise relative position and the less reliable, absolute position information.
However, JP 2009-198318 obtains a signal of a different period from an incremental pattern on each track, and thus the obtained phase accuracy of the incremental signal may lower due to mounting height offsets between the light source and the light receiving element, and an image magnification error caused by an inclination between the scale and the light receiving element. As a result, the position of the scale cannot be precisely detected.